1INIT(8) Linux System Administrator's Manual INIT(8)
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6 init, telinit - process control initialization
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9 /sbin/init [ -a ] [ -s ] [ -b ] [ -z xxx ] [ 0123456Ss ]
10 /sbin/telinit [ -t sec ] [ 0123456sSQqabcUu ]
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13 Init
14 Init is the parent of all processes. Its primary role is to create
15 processes from a script stored in the file /etc/inittab (see init‐
16 tab(5)). This file usually has entries which cause init to spawn get‐
17 tys on each line that users can log in. It also controls autonomous
18 processes required by any particular system.
19
21 A runlevel is a software configuration of the system which allows only
22 a selected group of processes to exist. The processes spawned by init
23 for each of these runlevels are defined in the /etc/inittab file. Init
24 can be in one of eight runlevels: 0–6 and S or s. The runlevel is
25 changed by having a privileged user run telinit, which sends appropri‐
26 ate signals to init, telling it which runlevel to change to.
27 Runlevels 0, 1, and 6 are reserved. Runlevel 0 is used to halt the sys‐
29 tem, runlevel 6 is used to reboot the system, and runlevel 1 is used to
30 get the system down into single user mode. Runlevel S is not really
31 meant to be used directly, but more for the scripts that are executed
32 when entering runlevel 1. For more information on this, see the man‐
33 pages for shutdown(8) and inittab(5).
34 Runlevels 7-9 are also valid, though not really documented. This is
36 because "traditional" Unix variants don't use them. In case you're
37 curious, runlevels S and s are in fact the same. Internally they are
38 aliases for the same runlevel.
39
41 After init is invoked as the last step of the kernel boot sequence, it
42 looks for the file /etc/inittab to see if there is an entry of the type
43 initdefault (see inittab(5)). The initdefault entry determines the ini‐
44 tial runlevel of the system. If there is no such entry (or no
45 /etc/inittab at all), a runlevel must be entered at the system console.
46 Runlevel S or s bring the system to single user mode and do not require
48 an /etc/inittab file. In single user mode, a root shell is opened on
49 /dev/console.
50 When entering single user mode, init initializes the consoles stty set‐
52 tings to sane values. Clocal mode is set. Hardware speed and handshak‐
53 ing are not changed.
54 When entering a multi-user mode for the first time, init performs the
56 boot and bootwait entries to allow file systems to be mounted before
57 users can log in. Then all entries matching the runlevel are pro‐
58 cessed.
59 When starting a new process, init first checks whether the file
61 /etc/initscript exists. If it does, it uses this script to start the
62 process.
63 Each time a child terminates, init records the fact and the reason it
65 died in /var/run/utmp and /var/log/wtmp, provided that these files
66 exist.
67
69 After it has spawned all of the processes specified, init waits for one
70 of its descendant processes to die, a powerfail signal, or until it is
71 signaled by telinit to change the system's runlevel. When one of the
72 above three conditions occurs, it re-examines the /etc/inittab file.
73 New entries can be added to this file at any time. However, init still
74 waits for one of the above three conditions to occur. To provide for
75 an instantaneous response, the telinit Q or q command can wake up init
76 to re-examine the /etc/inittab file.
77 If init is not in single user mode and receives a powerfail signal
79 (SIGPWR), it reads the file /etc/powerstatus. It then starts a command
80 based on the contents of this file:
81 F(AIL) Power is failing, UPS is providing the power. Execute the power‐
83 wait and powerfail entries.
84 O(K) The power has been restored, execute the powerokwait entries.
86 L(OW) The power is failing and the UPS has a low battery. Execute the
88 powerfailnow entries.
89 If /etc/powerstatus doesn't exist or contains anything else then the
91 letters F, O or L, init will behave as if it has read the letter F.
92 Usage of SIGPWR and /etc/powerstatus is discouraged. Someone wanting to
94 interact with init should use the /dev/initctl control channel - see
95 the source code of the sysvinit package for more documentation about
96 this.
97 When init is requested to change the runlevel, it sends the warning
99 signal SIGTERM to all processes that are undefined in the new runlevel.
100 It then waits 5 seconds before forcibly terminating these processes via
101 the SIGKILL signal. Note that init assumes that all these processes
102 (and their descendants) remain in the same process group which init
103 originally created for them. If any process changes its process group
104 affiliation it will not receive these signals. Such processes need to
105 be terminated separately.
106
108 /sbin/telinit is linked to /sbin/init. It takes a one-character argu‐
109 ment and signals init to perform the appropriate action. The following
110 arguments serve as directives to telinit:
111 0,1,2,3,4,5 or 6
113 tell init to switch to the specified run level.
114 a,b,c tell init to process only those /etc/inittab file entries having
116 runlevel a,b or c.
117 Q or q tell init to re-examine the /etc/inittab file.
119 S or s tell init to switch to single user mode.
121 U or u tell init to re-execute itself (preserving the state). No re-
123 examining of /etc/inittab file happens. Run level should be one
124 of Ss12345, otherwise request would be silently ignored.
125 telinit can also tell init how long it should wait between sending pro‐
127 cesses the SIGTERM and SIGKILL signals. The default is 5 seconds, but
128 this can be changed with the -t sec option.
129 telinit can be invoked only by users with appropriate privileges.
131 The init binary checks if it is init or telinit by looking at its
133 process id; the real init's process id is always 1. From this it fol‐
134 lows that instead of calling telinit one can also just use init instead
135 as a shortcut.
136
138 Init sets the following environment variables for all its children:
139 PATH /bin:/usr/bin:/sbin:/usr/sbin
141 INIT_VERSION
143 As the name says. Useful to determine if a script runs directly
144 from init.
145 RUNLEVEL
147 The current system runlevel.
148 PREVLEVEL
150 The previous runlevel (useful after a runlevel switch).
151 CONSOLE
153 The system console. This is really inherited from the kernel;
154 however if it is not set init will set it to /dev/console by
155 default.
156
158 It is possible to pass a number of flags to init from the boot monitor
159 (eg. LILO). Init accepts the following flags:
160 -s, S, single
162 Single user mode boot. In this mode /etc/inittab is examined and
163 the bootup rc scripts are usually run before the single user mode
164 shell is started.
165 1-5 Runlevel to boot into.
167 -b, emergency
169 Boot directly into a single user shell without running any other
170 startup scripts.
171 -a, auto
173 The LILO boot loader adds the word "auto" to the command line if
174 it booted the kernel with the default command line (without user
175 intervention). If this is found init sets the "AUTOBOOT" environ‐
176 ment variable to "yes". Note that you cannot use this for any
177 security measures - of course the user could specify "auto" or -a
178 on the command line manually.
179 -z xxx
181 The argument to -z is ignored. You can use this to expand the com‐
182 mand line a bit, so that it takes some more space on the stack.
183 Init can then manipulate the command line so that ps(1) shows the
184 current runlevel.
185
187 Init listens on a fifo in /dev, /dev/initctl, for messages. Telinit
188 uses this to communicate with init. The interface is not very well doc‐
189 umented or finished. Those interested should study the initreq.h file
190 in the src/ subdirectory of the init source code tar archive.
191
193 Init reacts to several signals:
194 SIGHUP
196 Has the same effect as telinit q.
197 SIGUSR1
199 On receipt of this signals, init closes and re-opens its control
200 fifo, /dev/initctl. Useful for bootscripts when /dev is remounted.
201 SIGINT
203 Normally the kernel sends this signal to init when CTRL-ALT-DEL is
204 pressed. It activates the ctrlaltdel action.
205 SIGWINCH
207 The kernel sends this signal when the KeyboardSignal key is hit.
208 It activates the kbrequest action.
209
212 Init is compatible with the System V init. It works closely together
213 with the scripts in the directories /etc/init.d and /etc/rc{run‐
214 level}.d. If your system uses this convention, there should be a
215 README file in the directory /etc/init.d explaining how these scripts
216 work.
217
219 /etc/inittab
220 /etc/initscript
221 /dev/console
222 /var/run/utmp
223 /var/log/wtmp
224 /dev/initctl
225
227 Init assumes that processes and descendants of processes remain in the
228 same process group which was originally created for them. If the pro‐
229 cesses change their group, init can't kill them and you may end up with
230 two processes reading from one terminal line.
231
233 If init finds that it is continuously respawning an entry more than 10
234 times in 2 minutes, it will assume that there is an error in the com‐
235 mand string, generate an error message on the system console, and
236 refuse to respawn this entry until either 5 minutes has elapsed or it
237 receives a signal. This prevents it from eating up system resources
238 when someone makes a typographical error in the /etc/inittab file or
239 the program for the entry is removed.
240
242 Miquel van Smoorenburg (miquels@cistron.nl), initial manual page by
243 Michael Haardt (u31b3hs@pool.informatik.rwth-aachen.de).
244
246 getty(1), login(1), sh(1), runlevel(8), shutdown(8), kill(1), init‐
247 tab(5), initscript(5), utmp(5)
248 29 Jul 2004 INIT(8)